The present invention relates to a process for purifying an octafluorocyclobutane, a process for preparing a high-purity octafluorocyclobutane, a high-purity octafluorocyclobutane, and uses thereof.
Heretofore, in the process of producing semiconductor devices, a gas etching for partially removing a thin-film material is performed for forming a circuit pattern which constitutes a semiconductor circuit. At the same time, removal of deposits using a cleaning gas is performed to remove a thin-film starting material deposited to the inside of a reactor during the thin film formation. One of useful etching or cleaning gases conventional for the production process of a semiconductor device is octafluorocyclobutane (hereinafter referred to as xe2x80x9cFC-C318xe2x80x9d).
On the other hand, to keep up with recent tendency toward higher performance, smaller size, higher density wiring of electrical or electronic equipment, the circuit patterns are becoming finer and in order to form a higher-precision circuit pattern by etching, use of a high-purity etching gas from which impurities are eliminated as much as possible is demanded. When an etching gas contains an impurity even a small amount, this may cause generation of a large width line during the formation of a fine pattern and increase of defects in the product having a high density integrated circuit.
Also in the process of removing adeposits using a cleaning gas, residual impurities in the production process of a semiconductor device after cleaning must be reduced as much as possible so as to provide a high-purity and high-quality device. For this purpose, a high-purity cleaning gas containing substantially no impurity is demanded.
With respect to the production process of FC-C318, for example, a method of purifying FC-C318 obtained as a by-product in the production of tetrafluoroethylene (hereinafter sometimes referred to as xe2x80x9cFC-1114xe2x80x9d) or hexafluoropropene (hereinafter sometimes referred to as xe2x80x9cFC-1216xe2x80x9d) is known.
However, these FC-1114 and FC-1216 each is produced by thermally decomposing chlorodifluoromethane (hereinafter sometimes referred to as xe2x80x9cHCFC-22xe2x80x9d) as described, for example, in EP451793 and many kinds of substances are produced by this thermal decomposition. The reaction product also contains unreacted HCFC-22 and many chlorine-containing compounds.
The boiling points of FC-C318 and respective compounds contained as impurities after the thermal decomposition of HCFC-22 are shown in Table 1. Among these, FC-1114 and FC-1216 as objective products and unreacted HCFC-22 can be mostly separated by distillation.
However, chlorofluorocarbons, particularly, 2-chloro-1,1,1,2,3,3,3-heptafluoropropane (hereinafter sometimes referred to as xe2x80x9cCFC-217baxe2x80x9d), 1-chloro-1,1,2,2,3,3,3-heptafluoropropane (hereinafter sometimes referred to as xe2x80x9cCFC-217caxe2x80x9d), 2-chloro-1,1,1,2-tetrafluoroethane (hereinafter sometimes referred to as xe2x80x9cHCFC-124xe2x80x9d), 1-chloro-1,1,2,2-tetrafluoroethane (hereinafter sometimes referred to as xe2x80x9cHCFC-124axe2x80x9d), 1,2-dichlorotetrafluoroethane (hereinafter sometimes referred to as xe2x80x9cCFC-114xe2x80x9d), FC-1216 and 1H-heptafluoropropane (hereinafter referred to as xe2x80x9cHFC-227caxe2x80x9d), have a boiling point close to the boiling point of FC-C318 and therefore, FC-C318 having an impurity concentration of 1 ppm by mass or less can be hardly obtained through separation by distillation.
Therefore, a purification method other than the separation by distillation, such as extractive distillation, membrane separation and adsorption separation, is being attempted.
However, the extractive distillation method has a problem in that the equipment costs highly and the process is cumbersome. The membrane separation method has a problem in that an appropriate and practical membrane having properties necessary for separating FC-C318 from impurities is not known, and purification to high purity, for example, such that the content of impurities in FC-C318 is 1 ppm by mass or less, is difficult.
Also, as shown in Table 2, there is almost no difference in the molecular size (calculated value at stable state structure) between FC-C318 and the impurity compounds, there is only a small difference in the boiling point between FC-C318 and impurity compounds as described above, and FC-C318 and impurities are approximated in the structure and physical properties. Therefore, separation of FC-C318 from impurity compounds to obtain a high-purity FC-C318 can be hardly attained by an adsorption separation method using a known adsorbent such as activated carbon, silica gel, zeolite (molecular sieves) and molecular sieving carbon (hereinafter referred to as xe2x80x9cMSCxe2x80x9d).
Among these, activated carbon is effective to adsorb and thereby remove FC-1216, which is one of impurities, but all other impurities including chlorine compounds cannot be separated.
Accordingly, in conventional purification methods, it is difficult to obtain FC-C318 reduced in the concentration of fluorocarbon impurities, particularly CFC-217ba, to 1 ppm by mass or less.
As a result of extensive investigations to solve these problems, the present inventors have found that when crude octafluorocyclobutane containing impurities such as fluorocarbon is contacted with an impurity decomposing agent containing an iron oxide and an alkaline earth metal compound and then with an adsorbent, these impurities can be substantially removed with ease.
More specifically, the present inventors have found a purification process of FC-C318, where FC-C318 containing fluorocarbon impurities such as CFC-217ba, CFC-217ca, HCFC-124, HCFC-124a, CFC-114, FC-1216 and HFC-227ca, particularly CFC-217ba, in a concentration of 10 to 10,000 ppm by mass is contacted with an impurity decomposing agent and further with an adsorbent and thereby these impurities can be reduced to less than 1 ppm by mass. The present invention has been accomplished based on this finding.
An object of the present invention is to solve the above-described problems in conventional techniques and provide a process for purifying an octafluorocyclobutane, where impurities can be substantially removed from a crude octafluorocyclobutane containing impurities. More specifically, an another object of the present invention is to provide a purification process capable of effectively removing CFC-217ba, which has been difficult to remove by conventional purification processes, and reducing the impurities such as fluorocarbon to less than 1 ppm by mass.
A further object of the present invention is to provide a process for preparing an octafluorocyclobutane, comprising the above-described purification steps, and also provide a high-purity octafluorocyclobutane and uses thereof.
The process for purifying an octafluorocyclobutane according to the present invention comprises the step of contacting a crude octafluorocyclobutane containing impurities with an impurity decomposing agent under elevated temperature (heating) and then with an adsorbent to substantially remove the impurities from the crude octafluorocyclobutane.
The impurity decomposing agent preferably comprises an iron oxide and an alkaline earth metal compound.
The iron oxide is preferably a ferric oxide. The ferric oxide is preferably a xcex3-iron hydroxide oxide and/or a xcex3-ferric oxide.
The alkaline earth meal compound is preferably at least one compound selected from the group consisting of oxides, hydroxides and carbonates of an alkaline earth metal of magnesium, calcium, strontium or barium.
The impurity decomposing agent preferably contains from 5 to 40% by mass of an iron oxide and from 60 to 95% by mass of an alkaline earth metal compound, based on the entire mass of the impurity decomposing agent.
The impurity decomposing agent is preferably a granule comprising a powder of an iron oxide having an average particle size of 100 xcexcm or less and a powder of an alkaline earth metal having an average particle size of 100 xcexcm or less.
The impurity decomposing agent is preferably a granule having an average particle size of 0.5 to 10 mm.
The crude octafluorocyclobutane is preferably contacted with the impurity decomposing agent at 250xc2x0 C. to 380xc2x0 C.
The adsorbent is preferably at least one member selected from the group consisting of activated carbon, molecular sieving carbon and activated coal.
The activated coal is preferably an activated coal obtained by a process comprising the steps of washing original coal with an acid and water (step 1), heating the original coal at 50 to 250xc2x0 C. in an inert gas stream to deoxidize and/or dehydrate the original coal (step 2), heating the original coal at 500 to 700xc2x0 C. in an inert gas stream to re-carbonizing the original coal (step 3), and heating the original coal at 700 to 900xc2x0 C. in a mixed gas stream containing an inert gas, carbon dioxide and steam to activate the original coal (step 4).
The original coal is preferably obtained by carbonizing at least one member selected from the group consisting of coconut-shell coal, coal, charcoal and tar pitch under heating at 400 to 600xc2x0 C.
The acid is preferably a mineral acid and has an acid concentration of preferably from 1 to 1,000 mol/m3.
The acid is preferably hydrochloric acid and/or sulfuric acid.
At the transfer from the step 2 to the step 3, the original coal from the step 2 is preferably heated to 500 to 700xc2x0 C. at 300 to 500xc2x0 C./hr in an inert gas stream.
At the transfer from the step 3 to the step 4, the original coal from the step 3 is preferably heated to 700 to 900xc2x0 C. at 100 to 200xc2x0 C./hr in an inert gas stream.
The mixed gas preferably contains from 50 to 89% by volume of inert gas, from 10 to 30% by volume of carbon dioxide and from 1 to 20% by volume of steam, based on the entire volume of the mixed gas.
After the step 4, the activated coal from the step 4 is preferably cooled to room temperature at 200 to 300xc2x0 C./hr in an inert gas stream.
The iodine adsorption quantity of the activated coal is preferably from 700 to 1,000 mg/g.
The total content of alkali metals contained in the activated coal is preferably 1,000 ppm or less.
The alkali metal is preferably potassium and the total content of potassium contained in the activated coal is preferably 500 ppm or less.
The crude octafluorocyclobutane preferably contains the impurities in an amount of 10 to 10,000 ppm by mass.
The impurity is preferably at least one fluorocarbon selected from the group consisting of 2-chloro-1,1,1,2,3,3,3-heptafluoropropane, 1-chloro-1,1,2,2,3,3,3-heptafluoropropane, 1-chloro-1,2,2,2-tetrafluoroethane, 1-chloro-1,1,2,2-tetrafluoroethane, 1,2-dichloro-1,1,2,2-tetrafluoroethane, hexafluoropropene and 1H-heptafluoropropane.
After the impurities are substantially removed, the concentration of impurities remaining in the octafluorocyclobutane is preferably less than 1 ppm by mass.
The process for preparing an octafluorocyclobutane according to the present invention comprises the steps of producing a crude octafluorocyclobutane containing impurities, and contacting the crude octafluorocyclobutane with an impurity decomposing agent under elevated temperature (heating) and then with an adsorbent to obtain an octafluorocyclobutane from which impurities are substantially removed.
The step of producing a crude octafluorocyclobutane containing impurities may be the thermal decomposition of chlorodifluoromethane. Also, the impurity is at least one fluorocarbon selected from the group consisting of 2-chloro-1,1,1,2,3,3,3-heptafluoropropane, 1-chloro-1,1,2,2,3,3,3-heptafluoropropane, 1-chloro-1,2,2,2-tetrafluoroethane, 1-chloro-1,1,2,2-tetrafluoroethane, 1,2-dichloro-1,1,2,2-tetrafluoroethane, hexafluoropropene and 1H-heptafluoropropane.
The octafluorocyclobutane according to the present invention is characterized by containing less than 0.0001% by mass of a fluorocarbon impurity and having a purity of 99.9999% by mass or more.
The fluorocarbon is at least one fluorocarbon selected from the group consisting of 2-chloro-1,1,1,2,3,3,3-heptafluoropropane, 1-chloro-1,1,2,2,3,3,3-heptafluoropropane, 1-chloro-1,2,2,2-tetrafluoroethane, 1-chloro-1,1,2,2-tetrafluoroethane, 1,2-dichloro-1,1,2,2-tetrafluoroethane, hexafluoropropene and 1H-heptafluoropropane.
The gas according to the present invention is characterized by comprising the above-described octafluorocyclobutane.
The etching gas according to the present invention is characterized by comprising the above-described gas.
The cleaning gas according to the present invention is characterized by comprising the above-described gas.